J.P. Morgan Chase & Co. said about 76 million households were affected by a cybersecurity attack on the bank this summer in one of the most sweeping disclosed breaches of a financial institution.

The largest U.S. bank by assets said the unknown attackers stole customers’ contact information—including names, email addresses, phone numbers and addresses. The breach, which was first disclosed in August and is still under investigation by the bank and law enforcement, extended to the bulk of the bank’s customer base, affecting...